This application is based on and claims the priority under 35 U.S.C. xc2xa7119 of German Patent Application 197 45 782.1, filed on Oct. 16, 1997, the entire disclosure of which is incorporated herein by reference.
The invention relates to a laser beam source for a directional infrared countermeasures (DIRCM) weapon system especially for use in the self-defense of an aircraft against a flying body such as a missile having an infrared (IR) seeking head.
The use of guided missiles for attacking aircraft, tanks, other military vehicles, and ground-based installations is well known. Such guided missiles especially include IR-seeking or heat-seeking missiles having an IR-seeking head that includes IR-sensors to seek out, locate, identify, and track the IR-signature of IR-emissions from the intended target aircraft or the like. By tracking and following the IR-emissions of the target body, the guided missile guides itself to the target body, even if the target body is moving. Such guided missiles further include semi-active missiles that seek out and track an infrared target mark that is projected by an infrared beam onto the target body. This target marking beam may be projected onto the target body by appropriate equipment provided on the guided missile itself or by equipment separate from the missile that is operated by personnel launching and directing the missile.
Various countermeasures for self-defense against such guided missiles are known. Particularly, directional infrared countermeasures (DIRCM) weapon systems are used for the self-defense of an aircraft by disorienting or disabling the guidance systems of the IR-seeking guided missiles. The countermeasures weapon systems achieve this by directing an infrared beam at the seeking head of the attacking missile, whereby the IR beam is intended to disorient or irreversibly destroy the IR detectors or associated circuitry provided in the target seeking head of the missile. In older DIRCM weapon systems, the IR beam was provided by the broad band IR radiation of IR lamps, that was bundled or collimated as well as possible into a relatively narrow directed beam. In modern DIRCM weapon systems, the IR beam is provided as a narrow band IR laser beam.
An example of such a modern DIRCM weapon system is known by the acronym FLASH, based on the German description xe2x80x9cFlugzeug-Selbstschutz mit Laser zur Abwehr von Lenkflugkxc3x6rpern mit Suchkxc3x6pfen hoher Leistungxe2x80x9d, or translated into English xe2x80x9caircraft self-defense with laser for defense against guided missiles with seeking heads of high performancexe2x80x9d. The FLASH system uses a pulsed laser beam of variable wavelength for irreversibly destroying the seeking head or particularly the IR detectors of the seeking head of a guided missile, in order to counterattack or defend against the missile. Descriptions of the FLASH system have been published by Rudolf Protz and Gunther Sepp in xe2x80x9cLaser Systems for Optical Countermeasuresxe2x80x9d, International Symposium on optronics and Defense, Paris, Dec. 3-5, 1996, and by Rudolf Protz and D. Wittmer in xe2x80x9cFLASHxe2x80x94ein Lasergestxc3xctztes DIRCM-System zum Selbstschutz von Flugzeugen gegen Flugkxc3x6rper mit optronischen Suchkxc3x6pfenxe2x80x9d (xe2x80x9cFLASHxe2x80x94a Laser-Supported DIRCM System for Self-Defense of Aircraft Against Missiles having Optronic Seeking Headsxe2x80x9d) in Eloka-Symposium, Mannheim,, Germany, Oct. 16-18, 1996.
Different types or classes of guided missiles typically have infrared seeking heads that operate or are sensitive in different wavelength ranges. For example, typical shoulder fired surface-to-air missiles using PbS detectors are sensitive in a wavelength range of 2 to 3 xcexcm, and using InSb detectors are sensitive in a wavelength range of 3 to 5 xcexcm. In contrast, anti-tank missiles typically use HgCdTe detectors and are sensitive in a wavelength range of 8 to 12 xcexcm. Semi-active missiles, i.e. missiles using so-called target marking, use detectors that are sensitive at a wavelength of 1.06 xcexcm. Thus, in order to be effective, the wavelength or wavelength range of the IR laser beam emitted by the DIRCM system must lie within or at least overlap the wavelength range that is transmitted through the respective seeking head optics system as well as the wavelength range in which the seeking head detectors themselves are sensitive. This requirement also pertains if the laser light reflected from the seeking head is to be analyzed for the purpose of more accurate target recognition and identification.
Suitable IR lamps or laser sources have previously not been available, especially in the wavelength range from 3 to 5 xcexcm. In order to provide IR radiation sources also in this wavelength range, the above mentioned publications suggest using a solid state laser with a fixed wavelength, for example a wavelength of 1.06 xcexcm for an Nd:YAG laser pumped by semiconductor laser diodes. Such a solid state laser is to be used as a pumping laser for exciting or driving optical parametric oscillators (OPOs). Such an OPO essentially comprises a nonlinear crystal arranged in an optical resonator, whereby the energy of a pumping photon is divided or distributed to two photons in the OPO. As a result, two laser beams, namely the so-called signal wave xcex and idler wave xcex* are generated.
In the OPO, the distribution ratio of the energy to the two photons, and therewith the respective wavelengths of the two laser beams being generated are determined by various parameters, and are dependent on the index of refraction of the OPO crystal for example. For a given crystal material, the index of refraction is dependent on the crystal temperature, the orientation of the crystal relative to the direction of the pumping laser beam, and the orientation of the crystal""s optical axis relative to the macroscopic OPO crystal or parallelepiped as determined when the finished crystal is being cut out of the initial crystal starting material. In this manner, dependent on the desired wavelength range, it is possible to achieve each desired wavelength within the required wavelength range by using proper crystals in respective one or two-stage OPOs. Such a method and system are described by F. Kenneth Hopkins in xe2x80x9cNonlinear Materials Extend the Range of High-Power Lasersxe2x80x9d in Laser Focus World, July, 1995, for example.
Previously suggested solutions for achieving the appropriate wavelength IR emissions require the use of several different radiation sources such as lamps, or different lasers, or different OPOs and the like in order to cover the respectively necessary wavelength range or ranges. For this reason, the known systems are not economically practical in view of their complex construction and operation.
Another disadvantage of the above described manner of generating the necessary laser beam using OPOs is that generally only a single respective wavelength will be available within the desired wavelength range. Namely, a second wavelength or plural wavelengths within the desired range are not provided. In this context, the second wavelength that is always simultaneously generated by the OPO system (i.e. xcex and xcex*) is generally not suitable for DIRCM purposes due to its inappropriate wavelength and intensity. This limitation of the known systems is a serious disadvantage. Namely, if the detector type of the seeking head to be counter-attacked is unknown, for example due to inadequate target recognition and identification, then the DIRCM IR beam must contain two or more wavelengths in various ones of the above mentioned wavelength ranges to be surely and reliably effective. In other words, if it is unknown in which wavelength range the seeking head sensors are sensitive, the DIRCM system must be able to emit suitable wavelengths in all of the possibly pertinent wavelength ranges. Previously, such a second wavelength could only be generated by providing at least one respective additional OPO with its own respective pumping laser. Such measures necessarily at least double the complexity, cost and size of the, system.
Still another disadvantage of previously known laser beam sources for DIRCM weapon systems is that the emitted laser beam having a fixed wavelength is subject to relatively simple countermeasures. Namely, the fixed wavelength of the emitted laser beam can be relatively easily determined by the attacker who launched the offensive missile, or this wavelength is already generally known to the attacker. Thus, the attacker can employ simple countermeasures, such as narrow band interference filters for example, which block out this particular wavelength. As a result, all of the DIRCM weapon systems employing a laser beam at a this particular wavelength will be ineffective.
In view of the above, it is an object of the invention to provide a laser beam source of the above discussed general type, which avoids the above disadvantages and which is able to generated a laser beam having a sufficient power, pulse frequency and spectral composition (e.g. having wavelength components at xcex=2.0 xcexcm and 4.0 xcexcm, or at xcex=2.1 xcexcm and 4.2 xcexcm, etc.), whereby the respective spectral components are adaptively selectable to meet the requirements at hand, for successfully countering or counter attacking various attacking flying bodies and particularly guided missiles having target seeking heads using different wavelength ranges. This is to be achieved throughout a counterattack engagement process including stages of target acquisition and identification, and employing defensive measures. In this context, it should be possible to carry out the corresponding selection of the appropriate beam parameters, namely the power, pulse frequency and spectral composition of the laser beam, both directly before, as well as during the defensive combating process, being carried by the DIRCM weapon system. In this manner, it should be possible to use any additional identifying information regarding the particular seeking head type of the attacking guided missile that the DIRCM weapon system can still acquire during the attack, for advantageously fine-tuning the counter combat procedures. The invention further aims to avoid or overcome the disadvantages of the prior art, and to achieve additional advantages, as apparent from the present description. The invention further aims to provide a method for carrying out countermeasures using such a DIRCM weapon system.
The above objects have been achieved using a laser beam source for a DIRCM weapon system according to the invention, and in a method of operating such a laser beam source according to the invention, wherein the laser beam source can be set to various different wavelength ranges both for the target acquisition and identification and also for the counter-combating measures and irreversible destruction of the detectors arranged in the target seeking head of the attacking guided missile. Particularly, the power, pulse frequency and selectable spectral composition of the laser beam is adjustable to match the particular requirements at hand.
The laser beam generator arrangement according to the invention comprises a semiconductor diode pumped laser as a pumping laser in combination with an optical parametric oscillator including a non-linear crystal. The crystal preferably comprises a plurality of different periodically polarized crystal zones arranged successively adjacent one another along the crystal, whereby the crystal zones respectively have different lattice spacings or lattice constants respectively correlated to the different wavelengths that are to be produced. By moving the crystal so that a selected one of the crystal zones is positioned in alignment with the pumping laser beam, it is possible to produce the desired output wavelength of the laser beam.
A further embodiment includes a plurality of crystal zones respectively grouped together into zone groups, whereby each zone group includes plural adjacent crystal zones, and the width of the zone group substantially corresponds to the beam cross-sectional width of the pumping laser beam so that the beam simultaneously impinges on all of the zones included within the respective group. Hereby, it is possible to produce a plurality of different output wavelengths simultaneously in the output beam. In this context, the relative intensities of the different wavelengths produced in the output beam correspond to the relative widths of the individual crystal zones within the respective group.